watchdog/imx_watchdog: do not set WCR_WDW

with WCR_WDW set, the watchdog won't trigger if we bootet linux and idle
around while the watchdog is not triggered. It seems the timer makes
progress very slowly if at all. I managed to remain 20minutes alive
while the timeout was set to 60secs. It reboots within 60secs if I start
a busyloop in userland (something like "while (1) { }").

While I don't see a reason why the WDT should not be running while the
CPU is in idle, I'm dropping this bit.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Stefano Babic <sbabic@denx.de>